There are two different techniques presently in use for detecting the fluid level in the fill-pipe of a fuel tank to provide automatic shut-off of the dispensing of the fuel in order to prevent overflow from the fill-pipe. Both of the currently used techniques rely on the basic principle of using a diaphragm to sense a pressure difference. In both cases, the pressure on one side of the diaphragm is a reference pressure, and it is maintained at atmospheric pressure by a passageway or port that is open to air surrounding the nozzle. The diaphragm moves when the pressure at a sensing port or passageway in the nozzle spout is different from that of the reference, and movement of the diaphragm controls shut-off of the fuel flow.
The first technique that is used to control the shut-off of fuel (gasoline, diesel, etc.) flow from a dispensing nozzle spout that is inserted into the fill-pipe of a tank of a motor vehicle or other fuel container relies on a Venturi shut-off mechanism. The second, more recent, technique for controlling fuel shut-off uses an electronic pressure switch. Although one mechanism is mechanical and the other is at least in part electronic, both rely on the action of a diaphragm as it responds to a pressure difference. The major distinction between the two control devices is that one depends upon a decrease in pressure being generated on one side of the diaphragm relative to atmospheric pressure on the other side, while the other depends upon a positive pressure increase on one side of the diaphragm relative to the atmospheric pressure on the other side, as will be described in detail below. What they have in common is that the sensing port is in the nozzle spout, near the tip, and gets covered by fuel as the tank fills. It is the covering of the sensing port that provides a sudden difference in pressure (positive in one case, negative in the other) across the diaphragm.
With the introduction of newly developed vapor recovery systems, particularly vacuum assist systems, sudden changes in the pressure in the fill-pipe region near the sensing port have been observed. Such changes affect the reliability of operation of a shut-off mechanism. It has become important to provide a more reliable shut-off mechanism.